N-protected-S-phenylcysteine (1) can be synthesized by, for example, treating a compound of the formula (2): ##STR3##
(wherein R.sup.1 and R.sup.2 are as defined above; X represents a leaving group) with thiophenol under basic conditions as described in Japanese Application Hei-10-264397. By way of illustration, N-benzyloxycarbonyl-S-phenylcysteine can be synthesized by treating N-benzyloxycarbonyl-.beta.-chloroalanine with thiophenol in an aqueous alkaline solution.
The N-protected-S-phenylcysteine (1) thus synthesized can be recovered typically by adding an acid to the basic aqueous solution thereof to precipitate the N-protected-S-phenylcysteine as a free acid. As a version of this technology, Japanese Kokai Publication Hei-10-29973 describes a process in which hydrochloric acid is added to an aqueous sodium hydroxide solution containing an N-benzyloxycarbonyl-S-phenylcysteine to cause the N-benzyloxycarbonyl-S-phenylcysteine to crystallize out as a free acid. In this process, the base salt of N-benzyloxycarbonyl-S-phenylcysteine (in this case, N-benzyloxycarbonyl-S-phenylcysteine sodium salt), which is readily soluble in water, is converted to the hardly-soluble free N-benzyloxycarbonyl-S-phenylcysteine by acidification. However, the process is not free from the disadvantage that, as pointed out in the same literature cited above, the free N-benzyloxycarbonyl-S-phenylcysteine tends to undergo gelation in the course of crystallization, and to fail in being crystallized in a good manner, in addition to the above problems, the process has also the problem that structurally related contaminant compounds cannot be easily removed. Thus, the methods heretofore known for crystallizing N-protected-S-phenylcysteine (1) as a free acid is not necessarily an effective isolation procedure.